Naoki, Daniel, Nutsinee

The newly installed squeezer (iLIGO) PMC took a while to lock so we investigated. Turned out the PMC had difficulty locking when the PZT was scanning from 0-100V but was able to lock reliably when the PZT turned around from 100-0V. We maybe seeing something we've seen before in the OPO where some element inside of the PMC heated up, expanded the cavity length against the PZT movement from 0-100V making it difficult to grab a lock. We triggered the PMC lock on the Refl diode and the channel we used has 2kHz sampling rate. Beckhoff trigger is running at 1kHz. The resonance passed by so quickly that sometimes if we're lucky the trigger would work but most of the time it didn't.

However, when scanning from 100-0V the cavity expansion help extended the period that light stored inside the cavity, making it easier to grab lock. Now that the PMC is better aligned, the effect became more severe.

The scan shows PMC refl and trans having a much narrower dip/peak when scaning from 0-100V compared to 100-0V.

We revert the scan from 0-100V to 100-0V so it starts from the top. Changes related to PMC scan has been accepted in the SDF.

In case of the OPO the cavity self-lock due to heating within the crystal. In case of PMC we have about ~100mW of light missing that can't be accounted for. We believe this amount of light maybe absorbed somewhere inside of the PMC.